Biomedical subjects
R J Barish
Publications and source records attributed to R J Barish.
Practical high-density shielding materials for medical linear accelerator rooms.
High-energy linear accelerators are replacing lower energy units in radiation therapy centers. Radiation protection requirements necessitate expensive reconstruction of existing treatment rooms to accommodate these new machines. We describe two shielding materials: one made by embedding small pieces of scrap steel in cement, and the other made with cast iron in cement. Both materials produce high-density barriers at low cost using standard construction methods.
Health physics concerns in commercial aviation.
Airline pilots and flight attendants are occupationally exposed to cosmic radiation. There has been uncertainty about how to handle the radiation protection requirements of their unique situation. Calculated dose equivalents associated with typical flight routes and crewmember work patterns have recently been published. The results show that flight attendants and pilots on ordinary subsonic aircraft can receive annual doses approaching 10 mSv y-1. I argue that flight crewmembers should receive specific education regarding the risks to their health from radiation exposure. I also argue that a suitable dosimeter system should be employed to provide crewmembers with information on their total doses. This is of particular importance for pregnant crewmembers who risk exceeding recommended fetal dose limits by working some routes during their pregnancy. In addition to airline crewmembers, business "frequent flyers" may receive significant occupational exposures while traveling. They too need appropriate education in order to assist them in assessing their risks from such exposures. Finally, flying during a large solar proton event would significantly increase the dose that would be received. During such an event, the total dose to the fetus of a pregnant crewmember or passenger might exceed the 0.5 mSv recommended monthly maximum. Warnings and action plans for these special circumstances should be improved.
Linac performance during short irradiations.
Explore the source record for details and available documents.
A new stereotactic X-ray knife.
For many years, the irradiation of small volumes of tissue in the brain to necrotizing doses has been investigated as a "non-invasive" alternative to neurosurgery. We propose a new system in which a precisely machined helmet serves as a multi-port focussed X-ray collimator when it is itself irradiated by a conventional medical linear accelerator run in the electron mode. When the collimator is attached to a stereotactic frame, the geometric accuracy of delivering small radiation fields to the brain is limited primarily by the accuracy of the stereotactic localization, and is relatively independent of the positional stability of the accelerator. Field sizes as small as two millimeters are readily achievable. The problem of low dose rate associated with these small fields is overcome by the use of high electron beam currents.
Shielding block mounting with double-stick tape.
We describe a system of mounting radiotherapy shielding blocks using a double-stick adhesive tape as an alternative to the screw-and-nut system generally employed. The method is easy to use and eliminates the dose perturbations that are caused by holes or slots in a blocking tray. More than 2 years of experience with this method has demonstrated its reliability in a clinical setting.
The carcinogenicity of radiation therapy.
Ionizing radiation as used for therapy for cancer is probably weakly carcinogenic at worst. The probability that cancers will be induced at a distance from the treatment volume is so small that it can only be inferred from experiences with large populations exposed to much higher radiation doses. The risk of cancer in and adjacent to the treatment volume also appears to be small, especially in adults. Intensive radiotherapy or radiotherapy of children 20 to 30 years ago appears to have induced secondary cancers in about 3 to 4 per cent of those treated, but modern practice has every expectation of reducing this incidence. No precise risk factor can be offered, but it seems likely that less than 3 to 4 per cent is a reasonable projection. The reason for the low carcinogenicity in the treatment volume probably lies in the fact that the irradiation dose is high and many cells are killed rather than transformed. The frequency of the induction of radiogenic cancer adjacent to or near the treatment volume is expected to vary according to the tissue exposed. It is not estimated to exceed a few per cent in the worst instances (for example, breast and thyroid gland) and is much less than 1 per cent in most tissues.
Patient immobilization with a low-temperature splint/brace material.
The authors describe a low-temperature plastic which is useful for the construction of immobilizing devices where beam orientation is critical.
Autoionography.
A technique has been established for the autoradiography of beta-emitting radionuclides distributed on thin-layer chromatograms. The technique is derived from medical ionography and produces an electrostatic latent image of the distribution of radioactivity by using a planeparallel ionization chamber filled with a suitable gas. The latent image is formed on a highly insulating plastic sheet which covers one electrode of the ionization chamber and is made visible by methods employed in Xerography or related electrophotographic processes. An introduction to the theoretical basis of autoionography is presented and results of application of the technique are shown for prototype imaging chambers operating at pressures of from 1 to 100 atmospheres. The present sensitivity of autoionography compares favorably with that reported in the recent literature for film or film-fluorographic methods. An advantage of ionography is the simplicity and very low cost involved in producing each ionograph.
A new method of beta autoradiography.
Explore the source record for details and available documents.
Saturation curve in gases of high atomic number at pressures up to 8 atm. Part II: Freon 13-B1, and mixtures of Freon with xenon and krypton.
The saturation curve has been studied in Freon 13-B1 (CF3Br) and in mixtures of Freon with xenon and krypton up to a pressure of 8 atm. The enhanced initial recombination due to the electron affinity of Freon has been evaluated and an empirical formula constructed that describes the fraction of current which escapes initial recombination over a wide range of voltages, pressures, and electrode spacings. After correction for this initial recombination, the general saturation curve for Freon and for mixtures of this gas with krypton and xenon has been derived and, again, convenient empirical formulae established which allow the current collection efficiency to be calculated for any given parameters within the range investigated. These formulae are of practical value in the design of image-forming ionization chambers.
Extended expression for tissue-maximum ratio fitted to the Varian Clinac-4 and Clinac-6 accelerators.
An expression which generates tissue maximum ratio (TMR) data for the Varian Clinac-4 and Clinac-6 accelerators is presented. This semiempirical expression can be programmed easily for radiotherapy computer systems or the new generation of scientific calculators. Our technique leads to good agreement with the published data, as demonstrated by a total average percent deviation of the measured TMR values of -0.115% for the Clinac-4 and -0.002% for the Clinac-6.
Development of optical process for accessing three-dimensional patient topology.
A commercial optical system developed by Solid Photography, Inc., has been used to generate precise paraffin--polyethylene tissue compensators. A program with the objective of constructing a medical facility has been undertaken to access patient topology for treatment planning, tissue-compensator fabrication, and patient positioning verification on the treatment table. Currently, patients are taken to a specially equipped studio where a series of microprocessor-controlled reticles are projected on the surface to be analyzed and photographs are taken automatically for computer processing. The topology data then drives a machine tool to sculpt the tissue compensators and may be transmitted via acoustic-coupled terminals to a treatment-planning computer at the Medical Center. The tissue compensators have been found to conform accurately to patient contours. At present, data bases can be generated to accommodate volumes of approximately 70,000 cm3.
Source-surface distance and source-axis distance beam data acquisition.
Radiation beam characteristics are usually obtained by scanning in a water phantom. Central axis depth dose data, beam profiles, and isodose curves are derived from these measurements. Certain treatment planning systems require that beam profiles be scanned isocentrically rather than at a constant source-surface distance. We have looked at profiles taken both ways, with appropriate field-size correction, and find no significant difference between them. This should simplify data acquisition in many cases.
Teletherapy beam characteristics: the first second.
The beam characteristics of radiotherapy treatment units are measured and recorded under circumstances in which transient effects are eliminated. Output measurements are done by averaging several readings. Isodose curves are obtained during a period of irradiation which is large compared with that used in patient treatment. When treatments are given, the therapy unit is assumed to deliver radiation beams that match the data acquired during calibration. We have looked at the energy, field uniformity, and output constancy typical of a number of these machines in their initial stages of operation. Although most can provide their "reference" beam characteristics in a short time, there are exceptions. The implications for patient treatment, port filming, and film dosimetry are discussed.
Long-term use of an isotope check source for verification of ion chamber calibration.
A radioactive check source is recommended for operational and constancy checks of dosimetry systems used for the calibration of therapeutic x-ray generators, including linear accelerators. Eight years of data have been analyzed for two ion chambers (and their associated electrometers) irradiated at fixed geometry in such a device. These dosimetry systems have also been calibrated every 2 years at a single Accredited Dosimetry Calibration Laboratory. Our analysis suggests that when a check source is used, and the results are consistent, the interval between formal calibrations can be lengthened.